This invention relates generally to regulated step down or buck power converters and, more particularly, to a step down controller employed in such power converters. Various ways of performing both step down power conversion and regulated power conversion are known in the prior art. Switched mode operation is a common power conversion technique. The devices which use the regulation information to control switching of the power switching devices in power converters are known as power controllers.
The primary purpose of a step down converter is to efficiently transfer energy from a relatively high input voltage source to a lower voltage output source. In the case of a regulated step down converter, the output voltage should remain substantially constant and insensitive to input voltage and loading variations. In general, two common techniques for implementing step down converters have been employed in prior art linear converters and switching converters.
Linear step down converters provide a constant current between the input source and the output source. The voltage difference between input and output is dissipated across a pass element (e.g. a transistor). One major disadvantage of linear step down regulators is that the power loss in the pass element tends to cause relatively inefficient energy transfer between the input voltage source and the output. One major advantage of linear step down regulators is low ripple voltage at the output source.
Switching step down converters typically consist of a reactive pass element (e.g. an inductor) and one or more switching devices. The purpose of the step down controller is to selectively turn the switching devices on and off in a manner such that the average current flow between the input and output source meets the average current requirements of the load while maintaining a constant output voltage. Since the reactive components and switching devices dissipate very little power, switching converters tend to transfer energy from the input to the output in a very efficient manner. One major disadvantage of switching regulators is relatively high output ripple voltage compared to linear regulators.
In general, a step down converter utilizing an inductive pass element can be operated in two different modes. In a converter operating in continuous conduction mode (CCM), the current flowing in the inductive pass element is continuously changing and the average DC current is relatively large compared to peak to peak variation in the current. In such CCM converters, the instantaneous current through the inductive component generally remains greater than zero. The advantage of a converter of this type is that high average currents can be achieved while maintaining relatively smaller peak-to-peak currents. This will typically result in lower output ripple voltage. The major disadvantage of CCM converters is that they tend to be relatively complex and difficult to stabilize.
In a converter operating in discontinuous conduction mode (DCM), the current flowing in the inductive pass element will typically start at zero current, ramp upward to some peak value, and periodically return to zero. Current is therefore transferred between the input and output sources in discrete current packets whose average current can be relatively predictable. The discontinuous nature of the energy transfer between input and output tends to greatly simplify the step down converter control scheme. The disadvantage of converters of this type is that output ripple voltage tends to be relatively high compared to step down converters operating in the CCM mode.
FIG. 1B illustrates typical instantaneous inductor current waveforms that might be observed in the prior art controller of FIG. 1A that utilizes a DCM step down technique. Under maximum load, the current through the inductive pass element continuously ramps between zero current and a fixed peak current. Under moderate and light loads, the current ramps from zero current to the same fixed peak current, ramps back down to zero current, and then maintains the inductor current at zero for a period of time. Under moderate and light load conditions, the average inductor current is reduced but the peak inductor current remains constant.
FIG. 2B illustrates the inductor current waveforms in the present DCM buck converter of FIG. 2A. Under maximum load current, the observed operation of the present invention is similar to that of prior art. However, under moderate and light load conditions, operation is significantly different. Under moderate load conditions the inductor current maintains continuous ramping of the current between zero and some peak value. However, unlike the prior art controller, the value of the peak inductor current is continuously decreased under loading conditions less than the maximum. The advantages of such a control scheme include lower output ripple voltage and improved converter efficiency.